Method of cementing well casing



Dec. 24, 1968 R A, MQRRls ET AL uETHoD oF CEMENTING WELL CASING 2 Sheets-Sheet l Filed June 29. 1967 Dec. 24, 1968 R. A. MORRIS ET AL METHOD OF CEMENTING WELL CASING 2 sheets-sheet z Filed June 29, 1967 INVENTORS. AMA/IMJ TEFL/72 United States Patent O 3,417,816 METHOD F CEMENTING WELL CASING Richard A. Morris, Monroeville, and Abraham J. Teplitz,

Penn Hills, Pa., assiguors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed June 29, 1967, Ser. No. 649,896 11 Claims. (Cl. 166-24) ABSTRACT 0F THE DISCLOSURE After the borehole of a well has been drilled to the desired depth, small diameter pipe having scratchers mounted `on its outer surface is run into the well and moved vigorously while circulating a liquid to remove drilling cuttings and mud filter cake from the borehole wall through the interval to be cemented. A liquid sealing material, ordinarily a hydraulic cement, is displaced down the tubing to fill the borehole of the well above the level of the interval to be cemented. Movement of the small diameter pipe is continued While the sealing liquid is displaced into the Well to reduce channeling of the sealing material. Thereafter, small diameter pipe is pulled from the Well and casing is lowered into the desired position in the well. The sealing liquid is then allowed to set to form a solid material. Setting of the sealing material may be triggered by heating or radiation.

This invention relates to wells penetrating subsurface formations and more particularly to a method of setting casing in such wells.

The term casing is used to designate relatively large diameter pipe which is set in the borehole of wells to support the borehole wall. The well casing is commonly set in place by a sealing material, ordinarily a hydraulic cement such as portland cement, that is displaced down the casing, out of its lower end, and upwardly into the annular space between the outer surface of the casing and the borehole wall. Production of fluids from subsurface formations is accomplished through perforations made in the casing and cement sheath after the casing has been cemented in place.

It is essential that a pressure-tight seal be made between the outer surface of the casing and the borehole wall through the interval of the pay zone to prevent contamination of fluids from the pay zone with undesirable fluids. Moreover, if an effective seal is not obtained, high pressures from the productive formations may be transmitted to equipment higher in the Well not designed to with sand such pressures With the consequent danger of a blowout.

It is important that an effective cementing job be performed in the first attempt. An incomplete filling and setting of cement in the annular space surrounding the casing will not be discovered until the casing has been perforated. If the cementing does not provide an effective seal, it is then necessary to call cementing equipment back to the well to attempt to squeeze cement through perforations in the casing into voids in the original cement sheath. Squeeze cementing is not a very satisfactory remedy. 1n addition to being difiicult and expensive, squeeze cementing often fails to correct the improper primary cementing job.

There are two principal causes of incomplete filling of the annular space surrounding the casing with cement. As a cement slurry is discharged from the bottom of the casing and flows upwardly through the annular space, it usually forms channels through the drilling mud present in the well from the drilling operations and thereby bypasses a substantial part of the annular space.

ICC

Another cause of inadequate cementing is the drilling mud filter cake present on the borehole wall at the end of the drilling of the well. Part of the filter cake is soft. When pressure within the well is reduced to a pressure lower than that in the formation for production of fluids from the pay zone, the soft part of the filter cake flows to the perforations and forms a channel in the annular space through which unwanted uids can flow into the well.

Several different techniques have been employed in an attempt to improve cementing operations. One technique used to reduce channeling is to displace the cement slurry through the annulus at a high rate of flow with the hope that the highly turbulent flow will eliminate channeling. Ordinarly, high cement velocities are not an effective way to eliminate channeling. Other attempts to remove the filter cake have usually involved attaching scratchers to the outer surface of the casing and either reciprocating or rotating the casing in the borehole to scratch the drilling mud filter cake from the borehole walls.

Reciprocation of casing with scratchers mounted on its outer surface to clean drilling mud filter cake from the borehole of a well is usually a worrisome and often an extremely hazarodous operation which drilling contractors and crews are reluctant to perform. The heavy Weight of long strings of casing severly stresses the drilling rigs and the upper joints of the casing. Friction between the large outer area of the casing and both the borehole wall and the drilling mud severely increases the stresses that occur during reciprocation and greatly increases the chances of the casing pulling apart. That wall friction also adds large torsional stresses to the casing if the scratching is accomplished by rotation of the casing. Sometimes the wall friction is so high that neither reciprocation or rotation of the casing is possible. If the casing should drop in the hole for any reason, a difficult, if not impossible, fishing job is required.

The large diameter of the casing and the small clearance between outer surface of the casing and the borehole wall cause a strong plunger effect on the drilling mud in the borehole as the casing is reciprocated. Pressure surges large enough to fracture underground formations are sometimes caused by reciprocation with a resultant loss of drilling mud from the hole. Any time the level of drilling mud in the hole is decreased, there is a danger of a blowout. Because of the dangers in scratching by movement of casing, drilling rig crews seldom scratch the borehole wall adequately to do an effective job of removing the drilling mud filter cake even though specifically instructed on how the scratching operation is tov be performed.

In the method of this invention after the borehole of a Well has been drilled 'by any desired means to the desired depth, a small diameter pipe having scratchers mounted on its outer surface is run into the well. The scratchers are spaced along the pipe to scratch the entire borehole wall throughout the interval to be cemented during reciprocation or rotation of the small diameter pipe. The small diameter pipe is reciprocated or rotated in the well while a liquid is circulated until returns substantially free of cuttings are obtained at the surface. A sealing material is pumped down through the small diameter pipe and discharged a short distance above the bottom of the borehole to displace drilling mud in the 'borehole upwardly in the well. The small diameter pipe is moved during displacement of the sealing liquid to continue scratching the wall of the Well and to break up tendencies of the sealing material to channel in the borehole while the cement is displaced into the borehole. The small diameter pipe is Withdrawn from the well,

and casing is lowered into the desired position and held in place until the sealing material sets. It is essential to this invention that the volume of sealing material discharged from the small diameter pipe be such that the upper level of the sealing material when the small diameter pipe is withdrawn from the hole is above the upper limit of the interval to be cemented. If necessary, the sealing material can be heated or otherwise treated after placement of the casing to accelerate setting.

In the drawings:

FIGURE l is a diagrammatic vertical section of the well during the scratching of the filter cake from the borehole wall.

FIGURE 2 is a vertical section of the lower end of the well during the displace-ment of sealing material into the well.

FIGURE 3 is a vertical section of the lower end of the well after delivery of the sealing material into the well.

FIGURE 4 is a vertical sectional view of the lower end of the well after casing has been run into the well and showing a heater in place to accelerate setting of the sealing material.

Referring to FIGURE 1 of the drawings, a well is shown with the well bore extending downwardly through a pay zone 4, through which casing is to be set, to the bottom of the well bore in a formation underlying the pay zone. The term pay zone is used to designate underground formations containing uids to be produced by the well. The pay zone may be a single stratum or more than one stratum separated by nonproductive strata. Surface casing 6 and intermediate casing 8 are cemented in place in the upper end of the well by cement sheaths 7 and 9, respectively. A blowout preventer 10 is mounted on the upper end of the casing, and a drilling rig 12 is in position over the well.

A small diameter pipe 16, yhereafter referred to as drill pipe, extends downwardly through the well bore to a level slightly above the bottom of the borehole. A check valve, not shown, in the lower end of drill pipe 16 prevents flow upwardly into the lower end of drill pipe 16. A plurality of scratchers 18 is mounted on the outer surface of the small diameter pipe through the interval of the portion of the well to be cemented. Scratchers 18 may be of conventional design actuated by reciprocation, rotation, or both, of the drill pipe. Centralizers 20 on the drill pipe 16 center the drill pipe in the borehole.

Scratchers 18 are mounted on the outside of the drill pipe 16 in a manner to insure scratching of the entire borehole wall over the interval to be cemented and a distance such as 50 to 100 feet beyond the boundaries of the pay zone to prevent ow from the pay zone through the annulus around the casing into formations exposed by the borehole. If the scratchers are actuated by reciprocation of the drill pipe, the vertical travel of the drill pipe during reciprocation should be slightly greater than the distance between the pipe joints to make sure that the entire surface of the borehole wall is scratched throughout the interval to be cemented. If the scratchers are operated by rotation of the drill pipe, the scratchers should overlap, that is, mounted in such a way as to cover entire interval to be scratched. A liquid is circulated down the drill pipe and upwardly through the annulus during the scratching operation to carry filter cake scratched from the borehole wall to the surface of the ground. Circulation and scratching are continued until the circulating liquid discharged at the wellhead is substantially free of cuttings. A typical scratching and circulation will continue for two to four hours.

It is essential to this invention that the small diameter pipe on which the scratchers are mounted have a diameterv not exceeding about 50 percent of the borehole diameter. The Ipreferred small diameter pipe is drill pipe. The small diameter pipe is effective in greatly reducing pressure surges that may occur during reciprocation and in eliminating the danger of parting of the string that occurs when casing is moved the extent required to remove the filter cake. Because drilling rig crews are accustomed to moving drill pipe rapidly in the borehole, both the sources of danger and the drilling rig crews fears are eliminated.

After a substantial absence of cuttings and mud filter cake in the liquid returned to the surface, a suitable liquid sealing material 26 is displaced down the drill pipe and out its lower end into the borehole. Suficient sealing liquid is displaced into the borehole to till the borehole with sealing liquid to a depth such that on withdrawal of the drill pipe 16, the level of the sealing liquid will be above the upper boundary of the pay zone 4. Movement of the drill pipe continues during the displacement of the sealing liquid. The scratchers mix the drilling mud in the borehole with the leading portion of sealing liquid to eliminate channeling of sealing liquid through the drilling mud. The large width of the annulus resulting from the small diameter of pipe 16 as compared with casing further reduces channeling of the sealing liquid. It is desirable t0 precede the sealing liquid with a small spacer, indicated by reference numeral 24 in FIGURE 2, of a liquid such as water to avoid contamination of the sealing liquid with drilling mud chemicals. A plug is pumped down the drill pipe behind the sealing material to indicate when all of the cement has been discharged from the drill pipe.

After lling the well with the sealing liquid to the desired depth, the drill pipe 16 is withdrawn from the well and casing 14 is run into the well to a position with its lower end at the desired level above the bottom of the borehole. As illustrated in FIGURE 4, casing 14 is provided with a cementing shoe 30 having a check valve in its lower end to prevent ow of iluids from the borehole of the well upwardly into the casing. In some instances, when casing of larger diameter is being set in the well and the casing would float in the cement 26 and overlying drilling mud, a llup shoe which allows limited ow into the casing when the pressure diferenti-al from the outside to the inside of the casing exceeds a pre., determined pressure may be used in place of the diagrammatically illustrated cementing shoe 30. Centralizers 29 on the outer surface of casing 14 center the casing in the borehole.

The sealing liquid used in this invention to fill the annular space surrounding the casing may be any of a number of different types as long as it can be converted to a solid lling the annulus after the casing is in place. For example, liquids that set to form synthetic resins can be used. Ordinarily, however, the sealing liquid is a hydraulic cement such as portland cement or pozzolan cement. Because of the time required after the cement has been placed in the borehole to withdraw the drill pipe 16 from the hole and to lower the casing to the desired position, it is essential that the cement be retarded to remain uid until after the casing is in place. An API thickening time of 24 hours Will usually be adequate. A method for determining thickening times of cement is described in API `Recommended Practice for Testing Oil-well Cements and Cement Additives (API RP 10B, 11th edition, January 1962). The type of retarder will `depend on the depth of the well and the type of cement used. Typical retarders are listed in U.S. Patents Nos. 3,140,269 and 3,234,154. An example of a cement composition that has been found to be satisfactory is 50 percent API Class A cement, 50 percent pozzolan cement, 3 percent Dowell D28 retarder, and 2 percent Dowell D60 tluid loss reducer. The concentration of retarder and fluid loss reducer is expressed in percent by weight of the total of dry API Class A and pozzolan Cements.

To avoid delay in completing the well, it is usually desirable to accelerate the setting of the sealing material once the casing is in place. One method of accelerating setting of the sealing liquid is to heat it to elevated temperatures. For this purpose a burner 32, illustrated in FIGURE 4, can be lowered into the well to the level of the pay zone 4 and a fuel burned to heat the sealing liquid surrounding casing 14 in the interval of pay zone 4 to a temperature at which it quickly sets. A suitable burner is described in Patent No. 3,254,721. A11 electric heater also could be used. At the stage of the cementing operation shown in FIGURE 4, the heated portion 28 of the cement adjacent the lower stratum of pay zone 4 has set while the portion of the cement opposite the upper stratum has not yet been heated to a temperature causing the cement to set. Triggering the setting of retarded cement also can be accomplished by the heat of chemical reaction and is described in U.S. Patents Nos. 3,171,480 and 3,189,089. If a resin-forming liquid such as an acrylonitrile monomer is used, polymerization can be accelerated by gamma-ray emanations.

By eliminating the hazards inherent in conventional cementing operations, this invention provides a method which will not be surreptitiously avoided by crews conducting cementing operations. Since the crews are accustomed to movement of drill pipe in the borehole, they have no objection to reciprocating or rotating drill pipe as extensively as required to remove the drilling mud ilter cake from the borehole wall. The increased space between the outer wall of the pipe and the walls of the well prevents damaging pressure surges and the continued movement of the drill pipe while the cement or other sealing liquid is displaced into the borehole prevents channeling of the sealing liquid through the drilling mud. Thus, this invention eliminates the principal causes of failure of primary cementing of casing in wells. Moreover, the highly retarded cement used in this invention can be held in the liquid state while the operators make sure all conditions are right before going in the hole with casing. If conditions are not right, the cement can be circulated from the hole and the cementing operation repeated after remedial procedures have been completed.

We claim:

1. A method for cementing casing in the borehole of a well comprising moving small diameter pipe having scratcher elements mounted thereon in the borehole of the well with simultaneous circulation of a liquid down the pipe and upwardly through the annulus surrounding the pipe to remove iilter cake and drilling cuttings from the borehole wall through the interval to be cemented, displacing a sealing liquid capable of `setting to form a solid material downwardly through the small diameter pipe into the bore hole wall while continuing the movement of the small diameter pipe, the volume of the sealing liquid being adequate to till the borehole above the upper limit of the interval to be cemented when the small diameter pipe is withdrawn, withdrawing the small diameter pipe, lowering casing into the borehole while the sealing liquid is Huid, and suspending the casing until the sealing liquid sets to form a solid material.

2. A method as set forth in claim 1 in which the small diameter pipe is drill pipe.

3. A method as set forth in claim 1 in which the small diameter pipe has a diameter less than about 50 percent of the borehole diameter.

4. A method as set forth in claim 1 in which the cir culation of liquid and movement of the small diameter pipe before displacement of the sealing liquid is continued until the liquid discharged from the upper end of the bore hole is substantially devoid of cuttings.

5. A method as set forth in claim 1 in which the sealing liquid is a slurry of a hydraulic cement.

6. A method as set forth in claim 1 in which the sealing liquid is a slurry containing portland cement retarded to have a thickening time of at least 24 hours.

7. A method as set forth in claim 1 in which after the casing is lowered into the borehole the sealing liquid is treated to accelerate conversion of the sealing liquid to a solid material.

8. A method as set forth in claim 1 in which the sealing liquid is a hydraulic cement and the hydraulic cement is heated after placement of the casing.

9. A method as set forth in claim 1 in which the sealing liquid is a resin-forming material and the sealing liquid is exposed to radiation to trigger setting after the casing is in place.

10. A method as set forth in claim 1 in which the small diameter tubing is reciprocated during the scratching and the displacement of sealing liquid.

11. A method as set forth in claim 1 in which the small diameter pipe is rotated during the circulation of liquid and the displacement of cement.

References Cited UNITED STATES PATENTS 1,866,522 7/1932 Jackson et al 166-25 2,801,715 v8/1957 Hall 166-27 3,302,715 2/ 1967 Smith et al. 166-25 DAVID H. BROWN, Primary Examiner.

U.S. Cl. X.R. l66-27, 33 

